1. Field of the Invention
The invention relates to long-term digital data storage media, and more specifically, to materials and manufacturing processes that produce very stable digital data storage media. In particular, an optical disc containing an inorganic nanomaterial data layer is disclosed.
2. Description of Related Art
One of the major issues with data archiving is media longevity. The data storage methods used today are insufficient for data storage beyond 50 years, 100 years, or longer. This longevity dilemma, as seen from the archivist's point of view, has many facets and equally many plausible, but unfortunately flawed potential solutions. For example, one method of perceived long-term data storage is optical digital data storage discs. Optical digital data storage comes in many capacities and formats, including, but not limited to the disc capacities of CD, Mini-Disc, DVD, HD, and BLU-RAY DISC® (BD) with dozens of format variations within each disc capacity, the most common including R, +R, −RW, +RW, RAM, to name a few. Given the nature of the construction of these media, they appear impervious to aging and often carry long-life expectancy claims. Unfortunately, experimental data on the life expectancy of these media types contradicts such generous life estimates provided by some manufacturers. (See, for example, Stability Comparison of Recordable Optical Discs—A Study of Error Rates in Harsh Conditions, J. Res. Natl. Inst. Stand. Technol. 109, 517-524 (2004)).
Another frequently attempted solution for solving the longevity dilemma for long-term (typically greater than 5-7 years) retention and storage of digital data is to preserve data on magnetic media such as tape or a hard disk, and then to renew the stored data by re-copying the data onto a new tape or hard drive or optical storage disc on a periodic basis. Variations on this theme can be played using optical data storage technology of various data densities and formats. Further, while certain searchability issues maybe mitigated by data transition to optical format, continually re-writing previously archived data is not a workable solution. Transferring archival data from one volatile format to another of similar or even greater susceptibility is error prone and inherently risky. (See, for example, “Storage expert warns of short life span for burned CDs,” John Blau, Computerworld Magazine, Jan. 10, 2006.) Cost is another facet of the problem. Archiving the amount of data generated by a company or other entity during any particular year may not be initially difficult or costly, but archival costs compound exponentially as the data from preceding years is repeatedly re-written to new media in addition to the integration of any new data.
The search for a solution to the longevity dilemma has led to the creation of new data storage technologies that focus on increasing a system's data storage recording rate and data density. Examples include: Oriented Nano-Structure (see U.S. Patent Application 2007/0195672 (published Aug. 23, 2007)), holographic (See U.S. Patent Application 2007/0216981 (published Sep. 20, 2007)), and multi-layer technology (See U.S. Patent Application 2007/0242592 (published Oct. 18, 2007)). In each case, the focus is on significantly higher data storage densities: approximately 150 gigabytes per disc for Oriented Nano-Structure, greater than 250 gigabytes per disc for holographic and approximately one terabyte per disc for multi-layer media technology. Furthermore, as will be discussed herein, neither expanded data capability, nor increased recording speeds have positively impacted this problem.
One issue with the technology path described above is that the new media capacities and formats suffer from the same age-degradation effects as the older media capacities and formats. The write methods embodied in the aforementioned technologies write the data in a similar means and use materials that are nearly identical to that of previous technology generations. The one significant change with each succeeding generation has been smaller feature sizes that permit higher data densities, but which also exacerbate age-degradation effects.
When introduced in 1964, writeable optical data storage devices used lasers to record an analog-wave signal in an ablatable write layer, usually made of a thin layer of aluminum or rhodium, on a Mylar-substrate filmstrip. (See, for example, U.S. Pat. No. 3,314,073). Unwritten portions were reflective and written portions were absorptive or transmissive to a read laser. Later patents by the same inventors suggested encoding digital holes in the ablatable layer (see, for example, U.S. Pat. No. 3,474,457), mounting the media on a drum (see, for example, U.S. Pat. No. 3,654,624), and increasing read and write reliability by adding surface defect and error checking (see, for example, U.S. Pat. No. 3,657,707). These high-energy data storage designs suffered in part because “[i]n selectively burning thick metal layer storage media with modulated laser energy, there is a tendency to burn or destroy the substrate on which the film or metal layer is coated.” (See U.S. Pat. No. 3,665,483, column 3). The writing methods suggested by these early patents had the further disadvantage of depositing the ablative metal material on the writing optics, thereby effectively contaminating the write system.
Most CD and DVD −/+R formats today include low bleachable-energy dyes. The writing process occurs when a write laser increases the dye's internal energy to such a point that an irreversible chemical reaction occurs that either bleaches the dye, making the dye transparent, or “burns” the dye, making the dye more opaque to a read laser. Optical disc manufacturers select dyes, in part, for their ability to be easily bleached or burned at relatively low activation energies. These low bleachable-energy dyes suffer from the same or greater age degradation kinetics as those described above. Discs manufactured using these dyes may become unreadable in as few as three to five years. Hence, existing, low-energy melting or bleaching write processes make most modern optical media inappropriate as an archiving medium. Write layers requiring little energy to record an optical mark also require little energy to modify unwritten portions by natural chemical, thermal or environmental forces anytime after the initial recording.
Most commercial CD, DVD, and BD media use organic dyes in their data layer. Organic dyes are widely available and inexpensive, but suffer from poor longevity. Dyes can be oxidized over time, losing their fluorescent properties. Dyes on optical media can also be “bleached” by the laser that is used during the normal course of reading the data from the media. Bleaching chemically alters dyes such that they no longer function and are not detectable.
Ideally, to make an optical media disk suitable for archive purposes, the materials, write methods, and manufacturing processes preferably have significant immunity to thermal and chemical kinetic aging processes. The materials preferably are not subject to the age degradation effects that may eventually cause chemical or mechanical breakdown. The write process preferably requires sufficient energy such that the write layers' written portions are permanently modified and the unwritten portions are not and will not be easily modified through aging or other deterioration processes. Thus, in an ideal media, the write layer is permanently modified as written portions are completely ablated or removed and unwritten portions are not removable or changeable except through high-power writing processes.
Doubly coated core-shell nanocrystals have been used to spell letters A, B, and C in a solid film (Chon, J. W. M. et al., Appl. Phys. Lett. 85(23): 5514-5516 (2004)). The authors suggested that the technology was demonstrated to be feasible for application in multilayered optical data storage. Multilayer letters were also spelled using a mixture of a quantum dot and an azo-dye polymer (Li, X. et al., Appl. Phys. Lett. 92: 066309 (2008)).
It is therefore an object of the present invention to provide an optical medium for digital data that is suitable for archive purposes. It is additionally an object of the present invention to provide optical digital data storage media that is durable for extended periods of time, with a minimal period of time being 100 years, and a desirable period of time being several thousand years. It is more specifically an object of the present invention to use write layers and other materials in the optical media that are not easily modified and thus retain their permanency for long periods of time. Another object of the present invention is to provide an optical digital data storage media with a write layer that is ablatable or permanently modified through the writing process and a substrate or other disc layers that are not adversely affected by the writing process. It is also an object of the present invention to provide an archive-quality optical media that is compatible with one or more of the existing, widespread digital storage technologies or that is backwards compatible with existing CD, DVD, or BD optical data storage formats.